* struct.c (scm_struct_ref, scm_struct_set_x): Use

* struct.c (scm_struct_ref, scm_struct_set_x): Use
scm_struct_i_n_words to get the number of fields, not
-scm_struct_n_extra_words.

On the route to fancier struct printing:
* struct.c (scm_print_struct): New function to print a structure.
Include "genio.h" to support it.  This function doesn't do
anything interesting right now, but I think it should be here
anyway.
* struct.h: Include "print.h" and add prototype for
scm_print_struct.
* print.c (scm_iprin1): Call scm_print_struct instead of trying to
print structures ourself.